This invention relates to a single pass color xerographic printing system with a single polygon, single optical system Raster Output Scanning (ROS) system, and, more particularly, to a dual wavelength laser diode source for the ROS which images the dual beams at a single station as closely spaced spots on a dual layer photoreceptor with each photoreceptor layer sensitive to only one of the two wavelengths or with one photoreceptor layer sensitive to both wavelengths and located beneath the other layer which absorbs one of the two wavelengths and permits only the second wavelength of light to reach the lower layer. Hence, although sensitive to both wavelengths the lower layer is only accessed by one of the two wavelengths.
In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image, onto a recording medium, usually plain paper. While other methods are known, the most common method of sweeping the laser beam is to deflect it from a rotating mirror. A multifaceted, rotating polygon mirror having a set of related optics can sweep the beam or sweep several beams simultaneously. Rotating polygon mirrors and their related optics are so common that they are generically referred to as ROSs (Raster Output Scanners), while printers that sweep several beams simultaneously are referred to as multispot printers.
When a xerographic printer prints in two or more colors, it requires a separate latent image for each color printed, called a system color. Color prints are currently produced by sequentially transferring overlapped images of each system color onto an intermediate transfer belt that is passed multiple times, once for each system color, over the photoreceptor. The built-up image is then transferred to a single recording medium and fused. Such printers are called multiple pass printers.
Conceptually, one can build up multiple colors on a photoreceptor or intermediate transfer belt that is passed through the system only once, in a single pass, by using a sequence of multiple xerographic stations, one for each system color. The built-up image on the photoreceptor or ITB can be transferred to a recording medium in a single pass. Additionally, tandem xerographic stations can sequentially transfer images directly to the recording medium in a single pass. Such a printer, called a multistation printer, would have a greater output than a multipass printer operating at the same raster sweep speed because the rasters for each color are operating simultaneously in the single pass printer. However, the introduction of multistation printers has been delayed by 1) cost problems, at least partially related to the cost of multiple xerographic stations and the associated ROSs, and 2) image quality problems, at least partially related to the difficulty of producing similar spots at each imaging station and subsequently registering (overlapping) the latent images on the photoreceptors, transfer mediums or recording mediums.
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to desired images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Modern business and computer needs often make it advantageous and desirable to reproduce or print originals which contain two or more colors. It is sometimes important that the copy reproduced or printed also contain two colors.
Several useful methods are known for making copies having plural colors. Some of these methods make high quality images, however, there is need for improvements. In particular, it is desirable to be able to print images having two or more highlight colors rather than being limited to a single highlight color. It is also desirable to be able to produce such images in a single pass of the photoreceptor or other charge retentive surface past the printing process areas or stations.
One method of producing images in plural (i.e. two colors, black and one highlight color) is disclosed in U.S. Pat. No. 3,013,890 to W. E. Bixby in which a charge pattern of either a positive or negative polarity is developed by a single, two-colored developer. The developer comprises a single carrier which supports both triboelectrically relatively positive and relatively negative toner. The positive toner is a first color and the negative toner is of a second color. The method develops positively charged image areas with the negative toner and develops negatively charged image areas with the positive toner. A two- color image occurs only when the charge pattern includes both positive and negative polarities.
Plural color development of charge patterns can be created by the method disclosed by F. A. Schwertz in U.S. Pat. No. 3,045,644. Charge patterns are developed of both a positive and negative polarity. The development system is a set of magnetic brushes, one of which applies relatively positive toner of a first color to the negatively charged areas of the charge pattern and the other of which applies relatively negative toner to the positively charged areas.
U.S. Pat. No. 3,816,115 to R. W. Gundlach and L. F. Bean discloses a method for forming a charge pattern having charged areas of a higher and lower strength of the same polarity. The charge pattern is produced by repetitively charging and imagewise exposing an overcoated xerographic plate to form a composite charge pattern.
As disclosed in U.S. Pat. No. 4,403,848, a multi-color printer uses an additive color process to provide either partial or full color copies. Multiple scanning beams, each modulated in accordance with distinct color image signals, are scanned across the printer's photoreceptor at relatively widely separated points, there being buffer means provided to control timing of the different color image signals to assure registration of the color images with one another. Each color image is developed prior to scanning of the photoreceptor by the next succeeding beam. Following developing of the last color image, the composite color image is transferred to a copy sheet. In an alternate embodiment, an input section for scanning color originals is provided. The color image signals output by the input section may then be used by the printing section to make full color copies of the original.
In tri-level, highlight color imaging, unlike conventional xerography as well as other printing processes, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor. These three voltage levels can be developed to print, for example, black, white, and a single color.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. This patent teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by the Gundlach patent, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V cad). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V dad (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V cad and V dad potentials, (typically 500 v) and is referred to as V white. The CAD developer is typically biased about 100 v closer to V cad than V white (about 600 v), and the DAD developer system is biased about 100 v closer to V dad than V white (about 400 v).
It is an object of this invention to provide a dual wavelength beam laser source for a single polygon, single optics, ROS for use in a single pass color xerographic unit.
It is another object of this invention to provide a dual wavelength sensitive, dual layer photoreceptor for use with a single polygon, single optics, ROS in a single pass color xerographic unit.
It is yet another object of this invention to provide a single pass three color xerographic printing system using a single imaging station.
It is another object of this invention to provide a dual wavelength xerographic printing system without the need for beam splitters or beam separation.